1. Field of the Invention
The present invention relates to an oxidation catalyst useful in purifying exhaust and waste gases and, more specifically, to an oxidation catalyst capable of converting carbon monoxide to carbon dioxide in the presence of sulfur compounds such as SO.sub.2.
It is well-known that waste gas and exhaust gas streams containing unburned hydrocarbons and carbon monoxide pollutants can be purified by catalytic oxidation of these pollutants to less noxious species, e.g., to carbon dioxide and water. This is accomplished by contacting the gas stream with an oxidation catalyst. One class of conventional oxidation catalysts includes catalytic materials that comprise one or more platinum group metals dispersed on a particulate refractory inorganic oxide support material such as particulate alumina. The catalytic material may be disposed in a catalyst bed or coated onto a carrier such as a ceramic honeycomb. While such catalysts are effective for oxidizing unburned hydrocarbons and carbon monoxide to less noxious species, they also oxidize sulfur compounds, notably SO.sub.2, to SO.sub.3, which is considered to be environmentally undesirable. For example, in the treatment of diesel engine exhaust and in the treatment of effluent from stationary sources, the presence of SO.sub.3 is associated with elevated formation of particulates.
2. Related Art
U.S. Pat. No. 5,145,825 to Deeba et al, dated Sep. 8, 1992, discloses an oxidation catalyst that resists degradation upon exposure to sulfur oxides and which comprises a Group VIII precious metal, e.g., platinum, palladium and/or rhodium, dispersed on refractory silica particles which have been coated with titania, zirconia or mixtures thereof. The catalyst is described as being useful in oxidizing SO.sub.2 to SO.sub.3.
U.S. Pat. No. 4,937,058 to Dupin et al, dated Jun. 26, 1990, discloses catalytic materials for treating H.sub.2 S and organo-sulfur compounds.
U.S. Pat. No. 5,061,464 to Cordonna, Jr. et al, dated Oct. 29, 1991, discloses a sulfur-tolerant platinum group metal catalyst in which the platinum group metal is dispersed on a support material that may comprise silica, zirconia, titania, zeolite and/or low surface area alpha-phase alumina. The catalyst is used for the removal of carbon monoxide and/or hydrocarbons from a gas stream.
Chen et al, in an article titled "Sulfur-Resistant Character of Titania-Supported Platinum Catalysts" published by Elsevier Science Publishers B.V. in 1990, teaches that a catalytic material comprising platinum dispersed on titania was found to be resistant to poisoning by H.sub.2 S.
It is known in the art to employ vanadia on alumina as a catalyst for the selective oxidation of H.sub.2 S to SO.sub.2 (see U.S. Pat. No. 4,427,576, to Dupin, dated Jan. 24, 1984, at column 1, line 65 through column 2, line 3, citing U.S. Pat. No. 4,092,404 to Hass, dated May 30, 1978). However, the use of vanadia is undesirable since vanadium compounds are considered to be toxic and therefore pose a health hazard to those who are exposed to them in the preparation of the catalyst material.